Routine surveillance has indicated the presence of H5 and N1 avian influenza subtypes in samples from two wild mute swans in Michigan, but testing has ruled out the possibility of this being the highly pathogenic H5N1 strain that has spread through birds in Asia, Europe and Africa, according to USDA and the Department of Interior. Test results thus far indicate this is low pathogenicity avian influenza, which poses no threat to human health.
The swans were sampled as part of the expanded avian influenza surveillance program. They were showing no signs of sickness, which suggests that this is low pathogenicity avian influenza. Additionally, genetic analysis of the virus conducted at USDA's National Veterinary Services laboratories in Ames, Iowa, suggests that it is similar to a low pathogenicity strain that has been found in North America.
It is possible that these birds were not infected with an H5N1 strain, but instead with two separate avian influenza viruses, one containing H5 and the other containing N1. The confirmatory testing underway at NVSL will clarify whether one or more strains of the virus are present, the specific subtype, as well as pathogenicity.
APHIS Director Ron DeHaven explains a second pathogenicity test injects the virus into eight baby chicks. If after 10 days six or more of the chicks show signs of disease or die, it is classified as a highly pathogenic virus. These results are expected within two weeks and will be made public when completed. It should be noted that wild birds are known to harbor many influenza viruses, and the finding of one or more of these viruses during routine testing is not unusual.
The swans were sampled August 8 at the Mouillee state game area located on the coast of Lake Erie in Monroe County, Michigan. The samples were taken by USDA Animal and Plant Health Inspection Service personnel as part of an expanded wild bird monitoring program. The Departments of Agriculture and Interior are working collaboratively with States to sample wild birds throughout the United States for the presence of highly pathogenic avian influenza.
Initial screening tests on the swan samples were conducted by Michigan State University's Diagnostic Center for Population and Animal Health - part of USDA's National Animal Health Laboratory Network. These tests indicated the presence of an H5 avian influenza virus. Confirmatory testing at NVSL confirmed the H5 and the N1. This testing also suggests, but has not yet confirmed, that this is low pathogenicity avian influenza.
Sue Haseltine, Department of Interior associate director for biology, explains over 8,000 tests have been conducted over the past two months by her department. Nearly 4,000 of those tests were done in Alaska on birds who migrate from Asia. USDA has conducted another 2,000 to 3,000 tests. There is not a confirmed figure on the number of tests conducted at the state level.
Low pathogenicity avian influenza commonly occurs in wild birds, where it typically causes only minor symptoms or no noticeable symptoms. These strains of the virus are not a human health concern. This includes LPAI H5N1, commonly referred to as the North American H5N1. This strain of low pathogenicity avian influenza is very different from the more severe HPAI H5N1 circulating overseas, which is commonly referred to as the Asian H5N1.
Evidence of LPAI H5N1 has been found on two occasions in wild birds in the United States. In 1975 and 1986, it was detected in wild ducks. These detections occurred as part of routine sampling. LPAI H5N1 has also been detected in Canada, most recently in 2005.